This invention relates to the field of audio frequency signal limiting, either for absolute peak protection, or for rms or averaging limiting as used in the various disciplines.
Audio signal handling systems frequently rely on some kind of signal limiter to act as either a safety check on peak excursion, or as a program loudness control. Peak limiters are often incorporated to protect recording and broadcasting media from effects of overmodulation. For this discussion, overmodulation may be taken to mean overcutting, overdrive, tape saturation, etc., depending on the application and the vernacular of that field of use. This relates to the condition where the absolute peak signal amplitude exceeds a defined level which may be an overload point, or a specified reference level such as 100 per cent modulation of a radio carrier.
So called "rms limiters" are used where the absolute peak amplitude is not of specific interest, but the relative volume level of the audio must be controlled or intensified.
Because signal limiters operate with very high transfer ratios (the ratio of input signal change above limit threshold to output signal change), use of signal limiting can cause unwanted sonic degradation under most operating conditions. These degradation effects are well known in the art. They are usually referred to as "pumping", "breathing", "hole punching", etc. Often, the degradation effects are dependent on the specific method of limiting employed, the amount or depth of limiting used in the application, etc.
Many schemes have been devised to make limiters that are relatively free of sonic degradation. One technique that has been variously applied is known as "multiband limiting". Through multiband limiting, effects of gain intermodulation (where the level of a quieter sound is modulated by the limiter acting upon a louder sound which is simultaneously present), are reduced by breaking up the audio frequency spectrum into several bands and limiting in the individual bands, then adding the limited band outputs together. Thus, loud sounds in one part of the spectrum will not cause reduction of sounds in another part of the spectrum under limiting conditions. The resulting audio is therefore more natural sounding after limiting.
A major problem with multiband limiting is resultant from the spectrum dissection, however. Since each band output contributes only a fractional component of the total output signal, the usual multiband limiter output level will not be well controlled for program audio with varying frequency spectrum content or bandwidth. One hundred percent output level will only be achieved when all of the bands have full output in the correct phase. Therefore, in normal applications with multiband limiters, consistently achieving 100 percent output level implies a need for either very deep program limiting, causing all bands to achieve a large output signal, or very wide program spectral bandwith filling all the bands to at least the limit threshold. Both conditions are unlikely occurences in practice. Usually, great limiting depth is impractical or undesirable due to sonic degradation as well. Thus, loud conventional multiband limiters usually require subsequent wideband limiting or clipping to obtain consistent audio level. This wideband process obviously negates the sonic advantages of the multiband process.
The present invention overcomes the fractional band summing problem of multiband limiters by automatically adjusting the limit threshold of the bands so that the total sum of the bands will equal one hundred percent of an output reference level, even if only one band of the plurality of bands contains a signal or if the relative phase of the band signals might not favor a maximum sum. An advantage of this principle, and which differentiates the method from effectively being the same process as wideband limiting after multiband limiting, is the characteristic that the automatic limiting threshold (ALT) method of this invention causes the minimum possible limiting in the process that will satisfy a 100 percent output rendition. In contrast, conventional multiband limiting followed by wideband limiting will maintain a constant limiting threshold, which will result in limiting activity of a constant nature for narrowband signals, followed by additional limiting by the wideband limiter necessary to "limit down" the varying multiband sum output level. Thus, the present invention achieves 100 percent output level for narrow band audio signals by using minimum limiting, while the conventional system achieves the same output by employing greater limiting depth.
Another way to understand the advantage of the present invention is to observe that, due to the "minimum limiting depth" rule which is followed in the ALT servo technique of multiband limiting, very consistent output level is achieved for widely varying audio signal characteristics with much fewer and less noticeable sonic side effects. Since typically less limit depth in each band is used to obtain the desired total wave limiting density, less audio equalizing effect is created by the hinging action of the multiple band limiting. Also, due to the action just described, it is possible to drive the ALT multiband limiter into greater limiting depth, as may be intentional or accidental in the various applications, before distortion or other side effects become objectionable.
There have been previous attempts to overcome the multiband consistency problem. Some such systems are described in U.S. Pat. Nos. 4,460,871, 4,208,548 and 4,249,042. The present invention represents a totally different method and approach.
Other references of interest include U.S. Pat. Nos. 3,488,604; 4,412,100; 4,371,842 and 4,495,643.